Longitudinally flexible stent

ABSTRACT

An intravascular stent especially suited for implanting in curved arterial portions. The stent retains longitudinal flexibility after expansion. The stent is formed of intertwined meander patterns forming triangular cells. The triangular cells are adapted to provide radial support, and also to provide longitudinal flexibility after expansion. The triangular cells provide increased coverage of a vessel wall. The stent can have different portions adapted to optimize radial support or to optimize longitudinal flexibility. Loops in the stent are disposed and adapted to cooperate so that after expansion of said stent within a curved lumen, the stent is curved and cells on the outside of the curve open in length, but narrow in width whereas cells on the inside of the curve shorten in length but thicken in width to maintain a density of stent element area which much more constant than otherwise between the inside and the outside of the curve. As a result, when the stent is coated with a medicine the more constant density of stent elements results in an even dose being applied to the inside wall of the lumen, avoiding the possibility that a toxic dose be supplied at one area while a less than effective dose is applied to another area.

RELATED APPLICATIONS

[0001] This application is a continuation-in-part of Ser. No. 09/795,794filed Feb. 28, 2001, which is a continuation-in-part of Ser. No.09/516,753 filed Mar. 1, 2000 and which also claims the priority ofProvisional Application No. 60/202,723, filed May 8, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates generally to stents, which areendoprostheses implanted into vessels within the body, such as bloodvessels, to support and hold open the vessels, or to secure and supportother endoprostheses in the vessels. In particular, the presentinvention relates to a stent which is longitudinally flexible before andafter expansion.

BACKGROUND OF THE INVENTION

[0003] Various stents are known in the art. Typically stents aregenerally tubular in shape, and are expandable from a relatively small,unexpanded diameter to a larger, expanded diameter. For implantation,the stent is typically mounted on the end of a catheter, with the stentbeing held on the catheter at its relatively small, unexpanded diameter.By the catheter, the unexpanded stent is directed through the lumen tothe intended implantation site. Once the stent is at the intendedimplantation site, it is expanded, typically either by an internalforce, for example by inflating a balloon on the inside of the stent, orby allowing the stent to self-expand, for example by removing a sleevefrom around a self-expanding stent, allowing the stent to expandoutwardly. In either case, the expanded stent resists the tendency ofthe vessel to narrow, thereby maintaining the vessel's patency.

[0004] U.S. Pat. No. 5,733,303 to Israel et al. (“'303”), which isexpressly incorporated by reference, shows a unique stent formed of atube having a patterned shape which has first and second meanderpatterns having axes extending in first and second directions. Thesecond meander patterns are intertwined with the first meander patternsto form flexible cells. Stents such as this one are very flexible intheir unexpanded state such that they can be tracked easily downtortuous lumens. Upon expansion, these stents provide excellent radialsupport, stability, and coverage of the vessel wall. These stents arealso conformable, in that they adapt to the shape of the vessel wallduring implantation. It is readily apparent that, by nature, when thestent shown, for example, in FIG. 8 thereof is expanded in a curvedlumen, cells on the outside of the curve open in length, but narrow inwidth whereas cells on the inside of the curve shorten in length butthicken in width to maintain a density of stent element area which muchmore constant than otherwise between the inside and the outside of thecurve.

[0005] One feature of stents with a cellular mesh design such as thisone, however, is that they have limited longitudinal flexibility afterexpansion, which may be a disadvantage in particular applications. Thislimited longitudinal flexibility may cause stress points at the end ofthe stent and along the length of the stent. Conventional mesh stentslike that shown in U.S. Pat. No. 4,733,665 may simply lack longitudinalflexibility, which is illustrated by FIG. 1, a schematic diagram of aconventional stent 202 in a curved vessel 204.

[0006] To implant a stent, it maybe delivered to a desired site by aballoon catheter when the stent is in an unexpanded state. The ballooncatheter is then inflated to expand the stent, affixing the stent intoplace. Due to the high inflation pressures of the balloon—up to 20atm—the balloon causes the curved vessel 204 and even a longitudinallyflexible stent to straighten when it is inflated. If the stent, becauseof the configuration of its mesh is or becomes relatively rigid afterexpansion, then the stent remains or tends to remain in the same orsubstantially the same shape after deflation of the balloon. However,the artery attempts to return to its natural curve (indicated by dashedlines) in FIG. 1 with reference to a conventional mesh stent. Themismatch between the natural curve of the artery and the straightenedsection of the artery with a stent may cause points of stressconcentration 206 at the ends of the stent and stress along the entirestent length. The coronary vasculature can impose additional stress onstents because the coronary vasculature moves relatively significantamounts with each heartbeat. For illustration purposes, the differencebetween the curve of the vessel and the straightened stent has beenexaggerated in FIG. 1.

[0007] U.S. Pat. No. 5,807,404 to Richter, which is expresslyincorporated by reference, shows another stent which is especiallysuited for implantation into curved arterial portions or osteal regions.This stent can include sections adjacent the end of the stent withgreater bending flexibility than the remaining axial length of thestent. While this modification at the end of the stent alleviates thestress at the end points, it does not eliminate the stress along theentire length of the stent.

[0008] Various stents are known that retain longitudinal flexibilityafter expansion. For example, U.S. Pat. Nos. 4,886,062 and 5,133,732 toWiktor (“the Wiktor '062 and '732 patents”) show various stents formedof wire wherein the wire is initially formed into a band of zig-zagsforming a serpentine pattern, and then the zig-zag band is coiled into ahelical stent. The stents are expanded by an internal force, for exampleby inflating a balloon.

[0009] The coiled zig-zag stents that are illustrated in FIGS. 1 through6 of the Wiktor '062 and '732 patents are longitudinally flexible bothin the expanded and unexpanded condition such that they can be trackedeasily down tortuous lumens and such that they conform relativelyclosely to the compliance of the vessel after deployment. While thesestents are flexible, they also have relatively unstable support afterexpansion. Furthermore, these stents leave large portions of the vesselwall uncovered, allowing tissue and plaque prolapse into the lumen ofthe vessel.

[0010] Thus, it is desired to have a stent which exhibits longitudinalflexibility before expansion such that it can easily be tracked downtortuous lumens and longitudinal flexibility after expansion such thatit can comply with the vessel's natural flexibility and curvature whilestill providing continuous, stable coverage of a vessel wall that willminimize tissue sag into the lumen.

SUMMARY OF THE INVENTION

[0011] Accordingly, an object of the invention is to provide a stentthat is longitudinally flexible before expansion so that it can easilybe tracked down tortuous vessels and remains longitudinally flexibleafter expansion such that it will substantially eliminate any stresspoints by complying with the vessel's flexibility and assuming thenatural curve of the vessel.

[0012] Another object of the present invention is to provide a stentthat is longitudinally flexible after delivery such that it flexesduring the cycles of the heartbeat to reduce cyclic stress at the endsof the stent and along the stent.

[0013] Another object of the present invention is to provide a stentwith a closed cell pattern such that it provides good coverage andsupport to a vessel wall after expansion.

[0014] Other advantages of the present invention will be apparent tothose skilled in the art.

[0015] In accordance with these objects, the stent of the presentinvention is formed to be a tube having a patterned shape which hasfirst and second meander patterns having axes extending in first andsecond direction wherein the second meander patterns are intertwinedwith the first meander patterns.

[0016] In accordance with one embodiment of the invention, theintertwined meander patterns form cells which have three points at whichthe first and second meander patterns meet each other, and which in thissense could be called triangular cells. These three cornered ortriangular cells are flexible about the longitudinal axis of the stentafter expansion. These triangular cells provide comparable scaffoldingand radial strength to that of cells formed by intertwined meanderpatterns which have four points at which the first and second patternsmeet each other, and which in this sense could be called square cells.

[0017] In another embodiment of the invention, bands of cells areprovided along the length of a stent. The bands of cells alternatebetween cells adapted predominantly to enhance radial support with cellsthat are adapted predominantly to enhance longitudinal flexibility afterexpansion.

[0018] In another embodiment of the invention, the first meanderpatterns are adapted to prevent any “flaring out” of loops of the firstmeander patterns during delivery of the stent.

[0019] A stent according to the invention retains the longitudinalflexibility associated with the '303 cellular stent in its unexpandedstate, and has increased longitudinal flexibility in the expanded state.The stent does so without sacrificing scaffolding—i.e. coverage of thevessel wall—or radial support.

[0020] In this and other embodiments, cells formed by the meanderpatterns are such that, when the expanded stent is bent while inside alumen, the cells on the outside of the curve open in length, but narrowin width whereas the cells on the inside of the curve shorten in lengthbut thicken in width so that the area of the cell, and the density ofthe struts, remains much more constant than otherwise. This results inmaintaining a more constant density of stent elements in contact withthe lumen, irrespective of location on the inside or outside of a curvedsection. In turn, when the stent is coated with a medicine, a more evendose is applied to the inside wall of the lumen, avoiding thepossibility that a toxic dose be supplied at one area while a less thaneffective dose is applied to another area.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 shows a schematic diagram of a conventional rigid stentdeployed in a curved lumen.

[0022]FIG. 2 shows a schematic diagram of a stent of the presentinvention deployed in a curved lumen.

[0023]FIG. 3 shows a pattern for a stent made in accordance with thepresent invention.

[0024]FIG. 4 shows an enlarged view of one cell of the pattern of FIG.3.

[0025]FIG. 5 shows a pattern for a stent made in accordance with thepresent invention;

[0026]FIG. 6 shows an enlarged view of one cell of the pattern of FIG.5.

[0027]FIG. 7 shows a pattern for a stent made in accordance with thepresent invention.

[0028]FIG. 8 shows an enlarged view of one cell used in the pattern ofFIG. 7.

[0029]FIG. 9 shows an enlarged view of another cell used in FIG. 7.

[0030]FIG. 10 shows a schematic comparison of a four cornered or “squarecell” and a three cornered or “triangular” cell of the presentinvention.

[0031]FIG. 11 shows a pattern for a stent constructed according to theprinciples of the invention which has variable geometry along itslength.

[0032]FIG. 12 shows another pattern for a stent constructed according tothe principles of the invention.

[0033]FIG. 13 shows another pattern for a stent constructed according tothe principles of the invention.

[0034]FIG. 14 shows the expansion of a portion of a horizontal meanderpattern built according to the principles of the invention.

[0035]FIG. 15 shows a view of the shape of single cell on the outside ofa curve superimposed on the same cell on the inside of a curve.

DETAILED DESCRIPTION OF THE INVENTION

[0036]FIG. 2 shows a schematic diagram of a longitudinally flexiblestent 208 of the present invention. The stent 208 may be delivered to acurved vessel 210 by a balloon catheter, and implanted in the artery byinflating the balloon. As described before, the balloon causes theartery to straighten upon inflation of the balloon. However, upondeflation of the balloon, the stent 208 assumes the natural curve of thevessel 210 because it is and remains longitudinally flexible afterexpansion. This reduces any potential stress points at the ends of thestent and along the length of the stent. Furthermore, because the stentis longitudinally flexible after expansion, the stent will flexlongitudinally with the vessel during the cycles caused by a heartbeat.This also reduces any cyclic stress at the ends of the stent and alongthe length of the stent.

[0037]FIG. 3 shows a pattern of a stent according to the presentinvention. This pattern may be constructed of known materials, and forexample stainless steel, but it is particularly suitable to beconstructed from NiTi. The pattern can be formed by etching a flat sheetof NiTi into the pattern shown. The flat sheet is formed into a stent byrolling the etched sheet into a tubular shape, and welding the edges ofthe sheet together to form a tubular stent. The details of this methodof forming the stent, which has certain advantages, are disclosed inU.S. Pat. Nos. 5,836,964 and 5,997,973, which are hereby expresslyincorporated by reference. Other methods known to those of skill in theart such as laser cutting a tube or etching a tube may also be used toconstruct a stent which uses the present invention. After formation intoa tubular shape, a NiTi stent is heat treated, as known by those skilledin the art, to take advantage of the shape memory characteristics ofNiTi and its superelasticity.

[0038] The pattern 300 is formed from a plurality of each of twoorthogonal meander patterns which patterns are intertwined with eachother. The term “meander pattern” is taken herein to describe a periodicpattern about a center line and “orthogonal meander patterns” arepatterns whose center lines are orthogonal to each other.

[0039] A meander pattern 301 is a vertical sinusoid having a verticalcenter line 302. It will be recognized that this is not a perfectsinusoid, but only an approximation thereof. Thus, as used herein, theterm sinusoid refers to a periodic pattern which varies positively andnegatively symmetrically about an axis; it need not be an exact sinefunction. A meander pattern 301 has two loops 304 and 306 per periodwherein loops 304 open to the right while loops 306 open to the left.Loops 304 and 306 share common members 308 and 310, where member 308joins one loop 304 to its following loop 306 and member 308 joins oneloop 306 to its following loop 304. The vertical sinusoid of meanderpattern 301 has a first frequency.

[0040] A meander pattern 312 (two of which have been shaded forreference) is a horizontal pattern having a horizontal center line 314.A horizontal meander pattern 312 also has loops labeled 316, 318, 320,322, and between the loops of a period is a section labeled 324. Lookedat another way, these loops are part of a vertical sinusoid 303 whichhas a higher frequency than that of the meander patterns 301. Verticalsinusoids 301 alternate with vertical sinusoids 303. Vertical sinusoids303 have a second frequency higher than the first frequency of thevertical meander patterns, i.e., sinusoids 301.

[0041] Vertical meander pattern 301 is provided in odd and even (o ande) versions which are 180° out of phase with each other. Thus, each leftopening loop 306 of meander pattern 301 o faces a right opening loop 304of meander pattern 301 e and a right opening loop 304 of meander pattern301 o faces a left opening loop 306 of meander pattern 301 e.

[0042] The horizontal meander pattern 312 is also provided in odd andeven forms. The straight sections 324 of the horizontal meander pattern312 e intersect with every third common member 310 of the even verticalmeander pattern 301 e. The straight sections 324 of the horizontalmeander pattern 312 o also intersect with every third common member 310of the odd vertical meander pattern 301. Viewed as vertical sinusoids303, alternating sinusoids 303 are intermittently coupled to the meanderpatterns 301. For example, between points 315 and 317, where verticalpattern 303 is coupled to vertical pattern 301 e, there are two loops306 and one loop 304 of vertical pattern 301 e and three loops 319 andtwo loops 321 of vertical pattern 303. This corresponds to two cycles ofpattern 301 e and 3 cycles of pattern 303. Similarly, between two pointsof coupling between vertical pattern 301 o and vertical pattern 303 aretwo loops 304 and one loop 306, again making two cycles. There will bethree loops 321 and two loops 319, again equal to three cycles ofpattern 303.

[0043] Since this embodiment of the stent is made of NiTi, and it isreboundable, it typically will be self-expanding. Upon expansion of thestent, the loops of the vertical meander patterns 301 open up in thevertical direction. This causes them to shorten in the horizontaldirection. The loops in the horizontal meander pattern 312 open up bothin the vertical direction and the horizontal direction, compensating forthe shortening of the loops of the vertical meander patterns.

[0044] It should be noted that the loops of the horizontal meanderpattern 312, which are the loops of the vertical pattern 303 in thepresent invention avoids foreshortening in a self-expanding stent in aparticularly effective manner. A self-expanding stent formed of ashape-memory alloy must be compressed from an expanded position to acompressed position for delivery. As shown in FIG. 7, because of theconfiguration of the loops 319 and 321 of the horizontal meander pattern312, when the stent is compressed from an expanded position 602 to acompressed position 604, the length 606 of the horizontal meanderpattern (width of the vertical pattern 330) naturally shrinks.Consequently, when the stent expands, the loops 319 and 321 elongate andcompensate for the shortening of the vertical meander patterns 301 e and301 o as the vertical meander patterns 301 e and 301 o expand. Incontrast, a horizontal meander pattern with such shapes as N-shapes willnot naturally shrink longitudinally when compressed from an expandedposition 608 to a compressed position 610, as illustrated in FIG. 14.

[0045] A stent formed from the pattern of FIG. 3 and made of NiTi isparticularly well suited for use in the carotid artery or other lumenssubject to an outside pressure. One reason is that because the stent isformed of NiTi, it is reboundable, which is a desirable property forstents placed in the carotid artery. The other reason is that the stentof FIG. 3 offers excellent scaffolding, which is particularly importantin the carotid artery. Scaffolding is especially important in thecarotid artery because dislodged particles in the artery may embolizeand cause a stroke.

[0046]FIG. 4 is an expanded view of one flexible cell 500 of the patternof FIG. 3. Each flexible cell 500 includes: a first member 501 having afirst end 502 and a second end 503; a second member 504 having a firstend 505 and a second end 506; a third member 507 having a first end 508and a second end 509; and a fourth member 510 having a first end 511 anda second end 512. The first end 502 of the first member 501 is joined tothe first end 505 of the second member 504 by a first curved member 535to form a first loop 550, the second end 506 of the second member 504 isjoined to the second end 509 of the third member 508 by a second curvedmember 536, and the first end 508 of the third member 507 is joined tothe first end 511 of the fourth member 510 by a third curved member 537to form a second loop 531. The first loop 530 defines a first angle 543.The second loop 531 defines a second angle 544. Each cell 500 alsoincludes a fifth member 513 having a first end 514 and a second end 515;a sixth member 516 having a first end 517 and a second end 518; aseventh member 519 having a first end 520 and a second end 521; aneighth member 522 having a first end 523 and a second end 524; a ninthmember 525 having a first end 526 and a second end 527; and a tenthmember having a first end 529 and a second end 530. The first end 514 ofthe fifth member 513 is joined to the second end 503 of the first member501 at second junction point 542, the second end 515 of the fifth member513 is joined to the second end 518 of the sixth member by a curvedmember 539 to form a third loop 532, the first end 517 of the sixthmember 516 is joined to the first end 520 of the seventh member 519 by afifth curved member 548, the second end 521 of the seventh member 519 isjoined to the second end 524 of the eighth member 522 at third junctionpoint 540 to form a fourth loop 533, the first end 523 of the eighthmember 522 is joined to the first end 526 of the ninth member 525 by asixth curved member 549, the second end 526 of the ninth member 525 isjoined to the second end 530 of the tenth member 528 by a seventh curvedmember 541 to form a fifth loop 534, and the first end 529 of the tenthmember 528 is joined to the second end 512 of the fourth member 510. Thethird loop 532 defines a third angle 545. The fourth loop 533 defines afourth angle 546. The fifth loop 534 defines a fifth angle 547.

[0047] In the embodiment shown in FIG. 4, the first member 501, thethird member 507, the sixth member 516, the eighth member 522, and thetenth member 528 have substantially the same angular orientation to thelongitudinal axis of the stent and the second member 504, the fourthmember 510, the fifth member 513, the seventh member 519, and the ninthmember 512 have substantially the same angular orientation to thelongitudinal axis of the stent. In the embodiment shown in FIG. 4, thelengths of the first, second, third and fourth members 501, 504, 507,510 are substantially equal. The lengths of the fifth, sixth, seventh,eighth, ninth and tenth members 513, 516, 519, 522, 525, 528 are alsosubstantially equal. Other embodiments where lengths of individualmembers are tailored for specific applications, materials ofconstruction or methods of delivery are also possible, and may bepreferable for them. It can be seen that each cell includes two cyclesof the lower frequency vertical pattern and three cycles of the higherfrequency vertical pattern.

[0048] The first, second, third, and fourth members 501, 504, 507, 510may have a width that is greater than the width of the fifth, sixth,seventh, eighth, ninth, and tenth members 513, 516, 519, 522, 525, 528in that cell. The differing widths of the first, second, third, andfourth members and the fifth, sixth, seventh, eighth, ninth, and tenthmembers with respect to each other contribute to the overall flexibilityand resistance to radial compression of the cell. The widths of thevarious members can be tailored for specific applications. For example,the ratio of width may be approximately 50-70%. The fifth, sixth,seventh, eighth, ninth, and tenth members may be optimized predominantlyto enable longitudinal flexibility, both before and after expansion,while the first, second, third, and fourth members may be optimizedpredominantly to enable sufficient resistance to radial compression tohold a vessel open. Although specific members may be optimized topredominantly enable a desired characteristic, all the portions of thecell interactively cooperate and contribute to the characteristics ofthe stent.

[0049]FIGS. 5 and 6 show a pattern and an expanded view of one cell ofan embodiment of the present invention which is specially adapted for astent made of stainless steel. The pattern is similar to the pattern ofFIGS. 3 and 4, and the same reference numerals are used to indicate thegenerally corresponding parts. The stents of the embodiment of FIGS. 5and 6 will normally be expanded by a balloon, in conventional fashion.

[0050] The embodiments of FIGS. 3 and 5 can also be viewed as being madeup of high frequency and low frequency vertical sinusoidal patterns orvertical loop containing sections which are arranged generally in thecircumferential direction and which are periodically interconnected.Thus, there is a first loop containing section with loops occurring at afirst frequency extending along line 301 and a second loop containingsection with also occurring at said first frequency extending along line302. A third loop containing section 303 extending along line 305 hasloops occurring at a second frequency that is higher than said firstfrequency. It is disposed between the first and second loop containingsections and alternately joined to the first and second loop containingsections. In the illustrated embodiment, the high frequency is in aratio of 3/2 to the low frequency. As noted above, the higher frequencyloop containing elements are smaller in width. The relative widths canbe selected so that the high frequency elements are crimpable to thesame diameter as the lower frequency elements. A stent according toclaim 4, wherein the higher frequency elements provide improvedflexibility.

[0051] Furthermore the high frequency vertical patterns of smaller widthresult in elements having a lower maximal strain. Specifically, thelower maximal strain is below the maximum strain without non-elasticdeformation for the material of the stent. In this embodiment where thestent is made of stainless steel the lower maximal strain is belowapproximately 0.4%, even for a 150° bend, as confirmed by finite elementanalysis. On the other hand, in a '303 type stent, for an equivalentamount of bending, exhibits a maximum strain of 8%. Thus, although theincreased flexibility of the stent of the present invention means that,in addition to conforming better to the curved lumen, it will bend witheach beat of the heart. The strain during heart beat happens 8,000,000times every year and cannot be much above elastic limit without thestent breaking. Since, embodiments of the present invention keep thestrain below the limit means that the stent of the present invention canbend with the lumen as the heart beats, for many years without breaking.

[0052] Also in this embodiment of the invention, for example, the secondloops 531 are made stronger by shortening the third and fourth members507, 510. This helps assure that the second loops do not “flare out”during delivery of the stent through tortuous anatomy. This “flaringout” is not a concern with NiTi stents which are covered by a sheathduring delivery.

[0053] Furthermore, the length of the members in this embodiment may beshorter than the length of the corresponding members in the embodimentillustrated in FIGS. 3 and 4. Typically, the amount of strain allowed ina self-expanding NiTi stent may be around 10%. In a stainless steelstent, the amount of strain allowed during the plastic deformation whichtake place, for example, during expansion, typically may be 20% orgreater. Therefore, to facilitate stents made of NiTi and stents made ofstainless steel expanding to comparable diameters, the members of theNiTi stent may be longer than the members of a stainless steel stent.

[0054] When the stent is within a curved lumen when it is expanded, thestent is curved as shown in FIG. 2 The result of this curving, for asingle cell 500, is shown in FIG. 15. The cells on the outside of thecurve open in length, but narrow in width whereas the cells on theinside of the curve shorten in length but thicken in width. As a result,the density of the members per unit of surface area remains closer towhat it is in an uncurved, expanded condition, both on the inside andoutside of the curve. Similarly, as can be seen from FIG. 15, the areaof the cell remains more constant than it would without compensation.This results in maintaining a more constant density of stent elements incontact with the lumen, irrespective of location on the inside oroutside of a curved section. In turn, when the stent is coated with amedicine, a more even dose is applied to the inside wall of the lumen,avoiding the possibility that a toxic dose be supplied at one area whilea less than effective dose is applied to another area. In some cases,the ratio between a toxic dose and an effective dose may be smaller than10:1.

[0055] Specifically, it can be appreciated that, in cells on the outsideof the curve at the connection points 542 and 538, the cell will open upincreasing the length of the cell. In addition, at the junction points535, 536, 537, 539, 540 and 542, the adjoining struts will come closerto each other, to cause the cell to become narrower in width, or in thecircumferential direction, compensating for the increase in length. Onthe inside of the curve, the longitudinal distances must decrease.Again, it is easy to see that the compression which occurs on the insideresults in the struts on either side of the junction points 542 and 538being squeezed closed and the width of the cell decreasing. At the sametime, at the junction points 535, 536, 537, 539, 540 and 542, the strutswill move further apart from each other and the cell becomes more narrowin length but thicker in width again providing compensation. Thus, inboth cases, the increase in one direction is compensated in the otherdirection to make the area more constant than it would have been withoutthe compensation.

[0056]FIG. 7 illustrates another aspect of the present invention. Thestent of FIG. 7 is also constructed from orthogonal meander patterns301, 302. The meander patterns form a series of interlocking cells 50,700 of two types. The first type of cell 50 is taught by U.S. Pat. No.5,733,303. These cells are arranged so that they form alternating bands704 of first type of cells 50 and bands 706 of the second type of cells700.

[0057] As seen in FIG. 8 and particularly with respect to the celllabeled for ease of description, each of the '303 cells 50 has a firstlongitudinal apex 100 and a second longitudinal end 78. Each cell 50also is provided with a first longitudinal end 77 and a secondlongitudinal apex 104 disposed at the second longitudinal end 78. Eachcell 50 also includes a first member 51 having a longitudinal componenthaving a first end 52 and a second end 53; a second member 54 having alongitudinal component having a first end 55 and a second end 56; athird member 57 having a longitudinal component having a first end 58and a second end 59; and a fourth member 60 having a longitudinalcomponent having a first end 61 and a second end 62. The stent alsoincludes a first loop or curved member 63 defining a first angle 64disposed between the first end 52 of the first member 51 and the firstend 55 of the second member 54. A second loop or curved member 65defining a second angle 66 is disposed between the second end 59 of thethird member 57 and the second end 62 of the fourth member 60 and isdisposed generally opposite to the first loop 63. A first flexiblecompensating member (or a section of a longitudinal meander pattern) 67having curved portion and two legs with a first end 68 and a second end69 is disposed between the first member 51 and the third member 57 withthe first end 68 of the first flexible compensating member 67 joined toand communicating with the second end 53 of the first member 51 and thesecond end 69 of the first flexible compensating member 67 joined to andcommunicating with the first end 58 of the third member 57. The firstend 68 and the second end 69 are disposed a variable longitudinaldistance 70 from each other. A second flexible compensating member (or,a section of a longitudinal meander pattern) 71 having a first end 72and a second end 73 is disposed between the second member 54 and thefourth member 60. The first end 72 of the second flexible compensatingmember 71 is joined to and communicates with the second end 56 of thesecond member 54 and the second end 73 of the second flexiblecompensating member 71 is joined to and communicates with the first end61 of the fourth member 60. The first end 72 and the second end 73 aredisposed a variable longitudinal distance 74 from each other. In thisembodiment, the first and second flexible compensating members, andparticularly the curved portion thereof, 67 and 71 are arcuate.

[0058] When curved stent is expanded while inside a lumen, also in thecase of the cells 50, cells on the outside of the curve open in length,but narrow in width whereas the cells on the inside of the curve shortenin length but thicken in width to provide a density of the members perunit of surface area that remains more constant between the inside andoutside of the curve.

[0059] Specifically, it can be appreciated that, in cells on the outsideof the curve the flexible connecting members 67 and 71 will open upincreasing the distances 70 and 74. In addition, the members 57 and 60will come closer to each other, as will members 51 and 54. This willfurther lengthen the cell. But at the same time it will become narrowerin width, or in the circumferential direction to compensate for theopening up of the flexible connector members 67 and 71. On the inside ofthe curve, the longitudinal distances must decrease. Again, if is easyto see that the compression which occurs on the inside results in theloops 67 and 71 being squeezed closed and the distances 70 and 74decreasing. At the same time, the members 57 and 60 and members 51 and54 will move further apart from each other and the longitudinalcomponents of members 57, 60, 51 and 54 will decrease. Thus, the cellbecomes narrower in length but thicker in width. Thus, in both cases,the increase in one direction is compensated in the other direction tomake the area more constant than it would have been without thecompensation.

[0060] The second type of cell 700 is illustrated in FIG. 9 and the samereference numerals are used to indicate generally corresponding areas ofthe cell. The apices 100, 104 of the second type of cell 700 are offsetcircumferentially. Also, each flexible compensating member 67, 71includes: a first portion or leg 79 with a first end 80 and a second end81; a second portion or leg 82 with a first end 83 and a second end 84;and a third portion or leg 85 with the first end 86 and a second end 87,with the second end 81 and the second end 84 being joined by a curvedmember and the first end 83 and the first end 86 being joined by acurved member. The first end of a flexible compensating member 67, 71 isthe same as the first end 80 of the first portion 79, and the second endof a flexible compensating member 67, 71 is the same as the second end87 of the third portion 85. A first area of inflection 88 is disposedbetween the second end 81 of the first portion 79 and the second end 84of the second portion 82 where the curved portion joining them lies. Asecond area of inflection 89 is disposed between the first end 83 of thesecond portion 82 and the first end 86 of the third portion 85 where thecurved portion joining them lies.

[0061] While FIG. 7 illustrates a pattern of alternating bands of cells,the stent may be optimized for a particular usage by tailoring theconfiguration of the bands. For example, the middle band of the secondtype of cells 700 may instead be formed of cells 50, or vice versa. Thesecond type of cells in FIG. 7 may also utilize the cell configurationsdescribed with respect to FIGS. 4 and 6. The cell configurations ofFIGS. 4 and 6 provide the advantage that they will not cause any torqueof one portion of the cell relative to another portion of the cell aboutthe longitudinal axis of the stent upon expansion, which may happen whenthe second type of cells 700 expand, a torque which could cause a stentto deform, and stick out.

[0062] As illustrated in FIG. 7, all of the flexible compensatingmembers are arranged so that the path of the flexible compensatingmembers, from left to right, travels in a generally downward direction.The cells 700 can also be arranged so that the flexible compensatingmembers in one band are arranged in a generally upward direction, andthe flexible compensating members in an adjacent band are arranged in agenerally downward direction. One skilled in the art can easily makethese modifications.

[0063]FIG. 10 is a schematic representation comparing the cells 804 ofthe present invention, which have three points where the intertwinedfirst and second meander patterns meet and are in that sense threecornered or triangular cells, with cells 802 of the '303 stent whichhave four points where the intertwined first and second meander patternsmeet and are in that sense four cornered or square cells. Moreparticularly, on the left side of FIG. 10, a pair of vertical meanderpatterns 806, 826 are joined by members 808, 810, 812 (which aresections of longitudinal meander patterns) to form a plurality of threecornered or triangular cells 804. By triangular cell, it is meant thatthere are three sections 810, 812, 814, each having loop portions andthree associated points 816, 818, 820 of their joining, forming eachcell.

[0064] On the right side of FIG. 10, a pair of vertical meander patterns822, 824 are joined together compensating members 828, 830, 832, 834(which are sections of a longitudinal meander) to form a plurality ofsquare cells 804. By square cell, it is meant that there are foursections, each having loop portions, and four associated points of theirjoining, forming each cell. For example, the shaded cell 802 is formedfrom four sections 832, 836, 830, 838, with four associated points oftheir joining 840, 842, 844, 846.

[0065] Both the square cell and the triangular cell have two kinds ofsections with loops. The first kind of loop containing section is formedfrom a vertical meander pattern and is optimized predominantly to enableradial support. The second kind of loop containing section is optimizedpredominantly to enable flexibility along the longitudinal axis of thestent. Although each loop containing section is optimized predominantlyto enable a desired characteristic of the stent, the sections areinterconnected and cooperate to define the characteristics of the stent.Therefore, the first kind of loop containing section contributes to thelongitudinal flexibility of the stent, and the second kind of loopcontaining section contributes to the radial support of the stent.

[0066] In the square cell 802, it can be seen that the second kind ofloop containing sections 830, 832 each have one inflection point 848,850. In the triangular cell, the loop containing sections 810, 812 eachhave two inflection point areas 852, 854, 856, 858. The higher number ofinflection points allows more freedom to deform after expansion of thestent and distributes the deformation over a longer section, thus,reducing the maximal strain along these loop containing sections.

[0067] Furthermore, it can be seen that a square cell 802 is generallymore elongated along the longitudinal axis of the stent than atriangular cell 804, which is generally more elongated along thecircumference of the stent. This also contributes to higher flexibilityafter expansion.

[0068] If the first meander patterns 806, 822, 824, 826 of both types ofcells are constructed identically and spaced apart by the same amount,the area of a triangular cell 804 is the same as a square cell 802. Thiscan be more readily understood with reference to a band of cells aroundthe circumference of a stent. Each band will encompass the same area,and each band will have the same number of cells. Accordingly, the areaof each cell in one band formed of square cells will be the same as thearea of each cell in another band formed of triangular cells.

[0069] Although the areas of the cells are equal, the perimeter of thetriangular cell is larger than the perimeter of the square cell.Therefore, in comparison to a square cell, a triangular cell offersincreased coverage of a vessel wall.

[0070] In the particular embodiments described above, the stent issubstantially uniform over its entire length. However, otherapplications where portions of the stent are adapted to providedifferent characteristics are also possible. For example, as shown inFIG. 11, a band of cells 850 may be designed to provide differentflexibility characteristics or different radial compressioncharacteristics than the remaining bands of cells by altering the widthsand lengths of the members making up that band. Or, the stent may beadapted to provide increased access to a side branch lumen by providingat least one cell 852 which is larger in size then the remaining cells,or by providing an entire band of cells 854 which are larger in sizethan the other bands of cells. Or, the stent may be designed to expandto different diameters along the length of the stent. The stent may alsobe treated after formation of the stent by coating the stent with amedicine, plating the stent with a protective material, plating thestent with a radiopaque material, or covering the stent with a material.

[0071]FIGS. 12 and 13 show alternative patterns for a stent constructedaccording to the principles of the present invention. The stent shown inFIG. 12 has two bands of cells 856 located at each of the proximal end860 and distal and 862. The cells that form the bands of cells 856located at the ends of the stent are '303 type cells. The remainingcells in the stent are the same as described with respect to the cells500 depicted in FIG. 6. The stent shown in FIG. 13 has alternating bandsof cells 864, 866, 868. The first type of band of cells 864 is composedof '303 type cells. The second and third types of bands of cells 866,868 are formed of the cells described with respect to the cells 500depicted in FIG. 4. Of course, any various combination of cells may beused in the present invention.

[0072] Thus, what is described is a longitudinally flexible stent thatutilizes a closed cell structure to provide excellent coverage of thevessel wall. The general concepts described herein can be utilized toform stents with different configurations than the particularembodiments described herein. For example, the general concepts can beused to form bifurcated stents. It will be appreciated by personsskilled in the art that the present invention is not limited to what hasbeen particularly shown and described above. Rather, the scope of thepresent invention is defined by the claims which follow.

What is claimed is:
 1. A stent for holding open a blood vesselcomprising: a. a first loop containing section, the first loopcontaining section arranged generally in the circumferential direction,the loops in said first loop containing section occurring at a firstfrequency; b. a second loop containing section, the second loopcontaining section arranged generally in the circumferential direction,the loops in said second loop containing section also occurring at saidfirst frequency; and c. a third loop containing section the third loopcontaining section, the loops in said third loop containing sectionoccurring at a second frequency that is higher than said firstfrequency, disposed in the generally circumferential space between saidfirst and second loop containing sections and alternately joined to saidfirst and second loop containing sections, d. wherein the loops in saidfirst, second and third loop containing sections are disposed andadapted to cooperate so that, when the expanded stent is in a curvedlumen, cells on the outside of the curve open in length, but narrowcircumferentially whereas cells on the inside of the curve shorten inlength but widen circumferentially.
 2. A stent according to claim 1wherein compensation, which occurs when cells on the outside of thecurve open in length, but narrow circumferentially and cells on theinside of the curve shorten in length but widen circumferentially,results in a more constant density of stent element area between theinside and the outside of the curve than if the cells on the outsideonly lengthened and cells on the inside only shortened.
 3. A stentaccording to claim 2, wherein said stent is coated with a medicine andsaid compensation results in a more even dose being applied to theinside wall of the lumen.
 4. A stent according to claim 1 whereincompensation, which occurs when cells on the outside of the curve openin length, but narrow circumferentially and cells on the inside of thecurve shorten in length but widen circumferentially, results in a moreconstant stent cell area between the inside and the outside of the curvethan if the cells on the outside only lengthened and cells on the insideonly shortened.
 5. A stent according to claim 4, wherein said stent iscoated with a medicine and said compensation results in a more even dosebeing applied to the inside wall of the lumen.
 6. A stent for widening avessel in the human body comprising: a. a plurality of firstcircumferential bands containing a pattern of loops at a firstfrequency; b. a plurality of second circumferential bands containing apattern of loops at a second frequency higher than said first frequency,alternating with said first circumferential bands and periodicallycoupled thereto to form cells, c. wherein loops in said bands aredisposed and adapted to cooperate so that, when the expanded stent is ina curved lumen, cells on the outside of the curve open in length, butnarrow circumferentially whereas cells on the inside of the curveshorten in length but widen circumferentially.
 7. A stent according toclaim 6 wherein compensation, which occurs when cells on the outside ofthe curve open in length, but narrow circumferentially and cells on theinside of the curve shorten in length but widen circumferentially,results in a more constant density of stent element area between theinside and the outside of the curve than if the cells on the outsideonly lengthened and cells on the inside only shortened.
 8. A stentaccording to claim 7, wherein said stent is coated with a medicine andsaid compensation results in a more even dose being applied to theinside wall of the lumen.
 9. A stent according to claim 4 whereincompensation, which occurs when cells on the outside of the curve openin length, but narrow circumferentially and cells on the inside of thecurve shorten in length but widen circumferentially, results in a moreconstant stent cell area between the inside and the outside of the curvethan if the cells on the outside only lengthened and cells on the insideonly shortened.
 10. A stent according to claim 9, wherein said stent iscoated with a medicine and said compensation results in a more even dosebeing applied to the inside wall of the lumen.
 11. A stent for holdingopen a blood vessel formed of a plurality of triangular cells, eachtriangular cell comprising: a. a first loop containing section, thefirst loop containing section arranged generally in the circumferentialdirection; b. a second loop containing section joined to the first loopcontaining section at a first junction point; and c. a third loopcontaining section joined to the first loop containing section at asecond junction point and joined to the second loop containing sectionat a third junction point, d. wherein loops in said cells are disposedand adapted to cooperate so that, when the expanded stent is in a curvedvessel, cells on the outside of the curve open in length, but narrowcircumferentially whereas cells on the inside of the curve shorten inlength but widen circumferentially.
 12. A stent according to claim 11wherein compensation, which occurs when cells on the outside of thecurve open in length, but narrow circumferentially and cells on theinside of the curve shorten in length but widen circumferentially,results in a more constant density of stent element area between theinside and the outside of the curve than if the cells on the outsideonly lengthened and cells on the inside only shortened.
 13. A stentaccording to claim 12, wherein said stent is coated with a medicine andsaid compensation results in a more even dose being applied to theinside wall of the lumen.
 14. A stent according to claim 11 whereincompensation, which occurs when cells on the outside of the curve openin length, but narrow circumferentially and cells on the inside of thecurve shorten in length but widen circumferentially, results in a moreconstant stent cell area between the inside and the outside of the curvethan if the cells on the outside only lengthened and cells on the insideonly shortened.
 15. A stent according to claim 14, wherein said stent iscoated with a medicine and said compensation results in a more even dosebeing applied to the inside wall of the lumen.
 16. A stent for wideninga vessel in the human body comprising: a. a plurality of first meanderpatterns; b. a plurality of second meander patterns intertwined with thefirst meander patterns to form triangular cells, said first meanderpatterns and said second meander patterns disposed and adapted tocooperate so that after expansion of said stent, when said stent isdispose in a curved vessel, cells on the outside of the curve open inlength, but narrow circumferentially whereas cells on the inside of thecurve shorten in length but widen circumferentially.
 17. A stentaccording to claim 16 wherein compensation, which occurs when cells onthe outside of the curve open in length, but narrow circumferentiallyand cells on the inside of the curve shorten in length but widencircumferentially, results in a more constant density of stent elementarea between the inside and the outside of the curve than if the cellson the outside only lengthened and cells on the inside only shortened.18. A stent according to claim 17, wherein said stent is coated with amedicine and said compensation results in a more even dose being appliedto the inside wall of the lumen.
 19. A stent according to claim 16wherein compensation, which occurs when cells on the outside of thecurve open in length, but narrow circumferentially and cells on theinside of the curve shorten in length but widen circumferentially,results in a more constant stent cell area between the inside and theoutside of the curve than if the cells on the outside only lengthenedand cells on the inside only shortened.
 20. A stent according to claim19, wherein said stent is coated with a medicine and said compensationresults in a more even dose being applied to the inside wall of thelumen.
 21. A multicellular stent for holding open a lumen, comprising:a. a plurality of even and odd vertical meander patterns, the oddvertical meander patterns being located between every two even verticalmeander patterns and being out of phase with the even vertical meanderpatterns, b. a plurality of even and odd horizontal meander patterns,the odd horizontal meander patterns being located between every two evenhorizontal meander patterns, c. wherein the vertical meander patternsare intertwined with the horizontal meander patterns to form a pluralityof triangular cells, d. wherein said horizontal meander patterns andsaid vertical meander patterns are disposed and adapted to cooperate sothat after expansion of said stent, when said stent is disposed in acurved lumen, cells on the outside of the curve open in length, butnarrow circumferentially whereas cells on the inside of the curveshorten in length but widen circumferentially.
 22. A stent according toclaim 21 wherein compensation, which occurs when cells on the outside ofthe curve open in length, but narrow circumferentially and cells on theinside of the curve shorten in length but widen circumferentially,results in a more constant density of stent element area between theinside and the outside of the curve than if the cells on the outsideonly lengthened and cells on the inside only shortened.
 23. A stentaccording to claim 22, wherein said stent is coated with a medicine andsaid compensation results in a more even dose being applied to theinside wall of the lumen.
 24. A stent according to claim 21 whereincompensation, which occurs when cells on the outside of the curve openin length, but narrow circumferentially and cells on the inside of thecurve shorten in length but widen circumferentially, results in a moreconstant stent cell area between the inside and the outside of the curvethan if the cells on the outside only lengthened and cells on the insideonly shortened.
 25. A stent according to claim 24, wherein said stent iscoated with a medicine and said compensation results in a more even dosebeing applied to the inside wall of the lumen.
 26. An expandable stentcomprising a plurality of enclosed flexible spaces, each of theplurality of enclosed flexible spaces including: a) a first memberhaving a first end and a second end; b) a second member having a firstend and a second end; c) a third member having a first end and a secondend; d) a fourth member having a first end and a second end; the firstend of the first member communicating with the first end of the secondmember, the second end of the second member communicating with thesecond end of the third member, and the first end of the third membercommunicating with the first end of the fourth member; e) the firstmember and the second member with the curved portion at their endsforming a first loop; f) the third member and the fourth member with thecurved portion at their ends forming a second loop; g) a fifth memberhaving a first end and a second end; h) a sixth member having a firstend and a second end; i) a seventh member having a first end and asecond end; j) an eighth member having a first end and a second end; k)a ninth member having a first end and a second end; and l) a tenthmember having a first end and a second end, the first end of the fifthmember communicating with the second end of the first member, the secondend of the fifth member communicating with the second end of the sixthmember, the first end of the sixth member communicating with the firstend of the seventh member, the second end of the seventh membercommunicating with the second end of the eighth member, the first end ofthe eighth member communicating with the first end of the ninth member,the second end of the ninth member communicating with the second end ofthe tenth member, and the first end of the of the tenth membercommunicating with the second end of the fourth member; m) the fifthmember and the sixth member with the curved portion at their endsforming a third loop; n) the seventh member and the eighth member withthe curved portion at their ends forming a fourth loop; and o) the ninthmember and the tenth member with the curved portion at their endsforming a fifth loop, wherein, when the expanded stent is in a curvedlumen, cells on the outside of the curve at communication points of thefirst and fifth and fourth and tenth members, the cell opens upincreasing the length of the cell and at each of the first through fifthloops, the adjoining members come closer to each other, to cause thecell to become narrower circumferentially and compensating for theincrease in length, whereas cells on the outside of the curve atcommunication points of the first and fifth and fourth and tenthmembers, the cell closes down decreasing the length of the cell and ateach of the first through fifth loops, the adjoining members move apart,to cause the cell to become wider circumferentially and compensate forthe decrease in length.
 27. A stent according to claim 26 wherein thecompensation which occurs on the outside of the curve and on the insideof the curve results in a more constant density of stent element areabetween the inside and the outside of the curve than if the cells on theoutside only lengthened and cells on the inside only shortened.
 28. Astent according to claim 27, wherein said stent is coated with amedicine and said compensation results in a more even dose being appliedto the inside wall of the lumen.
 29. A stent according to claim 26wherein the compensation which occurs on the outside of the curve and onthe inside of the curve results in a more constant stent area betweenthe inside and the outside of the curve than if the cells on the outsideonly lengthened and cells on the inside only shortened.
 30. A stentaccording to claim 29, wherein said stent is coated with a medicine andsaid compensation results in a more even dose being applied to theinside wall of the lumen.
 31. A multicellular stent comprising: aplurality of bands of square cells, each square cell including a firstloop disposed generally longitudinally opposite a second loop, and firstpair of flexible compensating members joined to the legs of the firstand second loops; a plurality of bands of triangular cells, eachtriangular cell comprising a first loop containing section arrangedgenerally in the circumferential direction, a second loop containingsection connected to the first loop containing section, and a third loopcontaining section connected to the first loop containing section andthe second loop containing section, and wherein loops in both square andtriangular cells are disposed and adapted to cooperate so that, when theexpanded stent is in a curved vessel, cells on the outside of the curveopen in length, but narrow circumferentially whereas cells on the insideof the curve shorten in length but widen circumferentially.
 32. Amulticellular stent according to claim 31 wherein each band of cells atthe ends of the stent are formed of square cells.
 33. A multicellularstent according to claim 31 wherein: each cell in the plurality of bandsof triangular cells includes a third loop disposed generallylongitudinally opposite a fourth loop and a second pair of flexiblemembers joined to the cell sections containing the third and fourthloops to form a cell, the bands of second cells interspersed with thebands of first cells, and the first loop and the second loop aresubstantially aligned along a longitudinal axis of the stent, andwherein the third loop and the fourth loop are offset along thelongitudinal axis.
 34. A multicellular stent according to claim 31wherein the bands of triangular cells are interspersed with the bands ofsquare cells to form the stent.
 35. A stent according to claim 31wherein compensation, which occurs when cells on the outside of thecurve open in length, but narrow circumferentially and cells on theinside of the curve shorten in length but widen circumferentially,results in a more constant density of stent element area between theinside and the outside of the curve than if the cells on the outsideonly lengthened and cells on the inside only shortened.
 36. A stentaccording to claim 35, wherein said stent is coated with a medicine andsaid compensation results in a more even dose being applied to theinside wall of the lumen.
 37. A stent according to any of claims 31,wherein compensation, which occurs when cells on the outside of thecurve open in length, but narrow circumferentially and cells on theinside of the curve shorten in length but widen circumferentially,results in a more constant stent cell area between the inside and theoutside of the curve than if the cells on the outside only lengthenedand cells on the inside only shortened.
 38. A stent according to claim37, wherein said stent is coated with a medicine and said compensationresults in a more even dose being applied to the inside wall of thelumen.
 39. A stent according to any claim 38 wherein said more even doseavoids the possibility that a toxic dose is supplied at one area while aless than effective dose is applied to another area.
 40. A stentaccording to claim 31, wherein said stent is a self expanding stent. 41.A stent according claim 31, wherein said stent is a balloon expandedstent.